1. Field of the Invention
The invention relates to adhesive coated electrical conductors, bonded coils, and methods for producing adhesive coated conductors and bonded coils.
2. Description of the Prior Art
The concentric high and low voltage windings of a core-form power transformer are subjected to tremendous forces during a short circuit condition. The outer or high voltage winding is subjected to a radial force in the outward direction and the inner or low voltage winding is subjected to a radial force in the inward direction. Taps and manufacturing variations result in the electrical centers of the windings, i.e., the midpoint of the ampere turn distribution across a coil or winding, to be offset in a direction parallel to the winding axis or longitudinal centerline. The fundamental force of radial repulsion effectively acts between the electrical centers, and when the electrical centers are offset, a force component exists which tends to move the high and low voltage windings in opposite axial directions. Short circuit currents may typically be 15 or more times normal full load current, and the short circuit force on a winding is proportional to the square of the current in the winding.
The short circuit forces are also increased due to the displacement of the first half cycle of current, which displacement is a function of the ratio of the resistance to the reactance of the transformer. This increase due to current displacement is 3 to 4 times the value of the force with symmetrical current. Therefore, the short circuit forces may be 800 to 1,000 times the forces existing during full load current. The axial component is typically in the range of 7 to 15%, and thus the force which attempts to move the windings axially apart is indeed substantial.
Power transformers must, therefore, be constructed to withstand the short circuit forces to which they may be subjected. High density pressboard, improved mechanical stabilization of cellulosic insulation, mechanical prestressing of coils, and other techniques, have been developed over the years to increase the mechanical strength of transformer windings. The coils or windings are tight when manufactured. The substantial amount of cellulose material distributed through the height of a coil makes it difficult to maintain the tight initial construction, but the techniques referred to have contributed greatly to achieving stable dimensions over the operating life of a power transformer.
Environmental and energy considerations have been an impetus to the development of electrostatic dry powder techniques for coating magnet wire, replacing the conventional solvent based materials. With solvent based materials, the wire is coated in a series of passes to achieve the desired build dimension, requiring a substantial amount of energy to drive off the solvent and then the airborne solvent becomes a pollution problem which is costly to deal with. The electrostatic dry powder technique uses no solvent, and the desired build is achieved in a single pass. The only energy required is that which is necessary to heat the wire to the fusing and curing temperature of the particulated deposit on the wire.
The use of the electrostatic dry powder technique is economically attractive for insulating the copper and aluminum strap or wire used in power transformers. The turn insulation presently used is a thin, thermally upgraded cellulosic or synthetic paper. The use of a thin coating of resinous insulation on the strap conductor of a power transformer would reduce the cost of the windings. It would also reduce the amount of cellulose or other compressible material across the coil length, resulting in a turn insulation structure which will not age during the normal thermal cycling of transformer apparatus, and insulation which will not contribute to loosening of the winding or coil through use since its compressibility is insignificant. Thus, the mechanical strength of the winding should not be deleteriously affected with age. Copending application Ser. No. 725,215, filed Sept. 22, 1976, entitled "Apparatus for Cleaning and Coating an Elongated Metallic Member," now U.S. Pat. No. 4,051,809, which is assigned to the same assignee as the present application, discloses apparatus for successfully coating copper and aluminum strap or wire, providing the very thin, uniform, void-free coatings which are essential to use of such insulation in a power transformer winding.
The development of successful electrostatic dry powder techniques for insulating copper and aluminum strap or wire, however, did not prove to be the only problem encountered in substituting a resinous coating for paper in insulating the turns of power transformers. Short circuit tests have shown that the very smooth glassy finish of the powder coated conductor detracts from the short circuit strength of the winding. The paper turn insulation provides a much higher coefficient of friction between adjacent conductor turns, and it adds substantially to the short circuit strength of the winding.
Adhesive overcoats have been applied to magnet wire and foil, such as disclosed in U.S. Pat. Nos. 3,412,354 and 3,504,104, respectively, for providing conductors which may be wound into coils, the turns of which are then bonded together to increase the mechanical strength of the coil. However, applying prior art adhesive overcoat techniques to resin coated copper or aluminum conductor adversely affects the cost and/or the space factor of a coil wound from such conductor. Thus it would be desirable to be able to use resin coated conductor, such as disclosed in the hereinbefore mentioned copending patent application, in a transformer winding, and to achieve the required mechanical strength without suffering offsetting disadvantages of higher cost and/or lower space factor.